Inverters serve for converting an electrical power generated by a generator, for example an electrical power generated by a photovoltaic generator in the form of an electrical DC current, into an AC current for feeding into an AC voltage grid. For this purpose, an inverter comprises an input-side DC-DC converter and an output-side inverter bridge in order to set, by means of power semiconductors used therein, an input voltage of the inverter in such a way that the maximum possible power is drawn from the generator by virtue of the latter being operated at its maximum power point (MPP), wherein the input voltage is tracked to possible changes in the MPP (MPP tracking, MPPT for short). In this case, as a result of losses arising in the components of the inverter, in particular as a result of switching losses in the power semiconductors, as a result of ohmic losses in other components, and as a result of magnetization reversal losses occurring in particular in inductors within the inverter, heating of the components of the inverter occurs, in particular heating of the power semiconductors and of the inductors, wherein the components may be damaged as a consequence of excessive heating.
Therefore, it is necessary to monitor temperatures within the inverter and, upon exceedance of limit values assigned to the temperatures, to reduce the power loss arising in the components, in particular by reducing the electrical power of the generator and thus also the electrical power converted by the inverter.
DE 102011007929 A1 discloses a method for downwardly regulating an inverter in which a first measurement variable, representative of a temperature within the inverter bridge, a second measurement variable, representative of an electrical power fed into an AC voltage grid, and further measurement variables are detected, wherein the further measurement variables may comprise a voltage at the generator terminals of the inverter, i.e. an input voltage of the inverter. Depending on the measurement variables, a reduced power value is determined, to which the electrical power fed into the AC voltage grid is reduced, in particular by a DC current generated by the generator being reduced to a maximum generator current value; in the case of a photovoltaic generator, this reduction of the DC current generated by the generator is performed on the basis of an increase in the input voltage of the inverter.
EP 0827254 A1 discloses a method for controlling a power of an inverter for a photovoltaic generator in which an input voltage of the inverter is regulated in such a way that a maximum possible electrical power is drawn from the photovoltaic generator, provided that it is below a limit value for the electrical power processed by the inverter. If the maximum possible electrical power of the photovoltaic generator exceeds the limit value, the input voltage of the inverter is increased and subsequently regulated in such a way that the power drawn from the photovoltaic generator corresponds to the limit value. As a result, the inverter is protected against overheating.
US 20130077372 A1 discloses a method for operating an inverter for a photovoltaic generator with an input-side DC-DC converter at an operating point with reduced power, in which the operating point with reduced power, if the MPP power of the photovoltaic generator exceeds a maximum permitted electrical power specified for the inverter, is set by an input voltage of the inverter which is below or above the MPP voltage of the photovoltaic generator being set by means of the DC-DC converter. In this case, an operating point with an input voltage below the MPP voltage is chosen if an open circuit voltage of the photovoltaic generator is above a maximum permitted input voltage of the inverter and a short-circuit current of the photovoltaic generator is below a maximum permitted input-side DC current; otherwise, an operating point with an input voltage above the MPP voltage is chosen. This method makes it possible to prevent in particular an overloading of the inverter and at the same time a switching-off of the inverter on account of an excessively high input voltage and/or on account of an excessively high DC current.
US 20090285998 discloses a method for operating an inverter, wherein the inverter is connected to an electrical generator on the input side via a DC-DC converter and an input voltage of the inverter corresponds, by means of MPP tracking, to an MPP voltage at which the generator outputs a maximum electrical power, wherein a temperature value in the DC-DC converter is determined and the MPP tracking is discontinued if the temperature value in the DC-DC converter exceeds a maximum permissible limit value.